There are ink compositions which are colorless and invisible under ordinary visible light but emit light and become visible in response to illumination of light at a specific wavelength such as ultraviolet light. In general, such an ink composition is called invisible ink. As a specific example of invisible ink, Japanese Patent Application Laid-Open No. 11-279474 (Patent Document 1) discloses an ink composition (see a chemical formula in FIG. 18) containing a fluorescent compound of arylsulfonic acid and tertiary amine.
An application of invisible ink is disclosed in Japanese Patent Application Laid-Open No. 2003-296659 (Patent Document 2) as a method of downloading information from a server on a network using a QR code as a kind of two-dimensional bar code written in invisible ink. According to an embodiment described in Japanese Patent Application Laid-Open No. 2003-296659, a publisher prints a QR code in an invisible ink (transparent material) absorbing infrared rays to overlap an article printed in visible black ink on a magazine. Then, a reader of the magazine uses a camera-equipped cellular phone to which infrared rays are emitted to recognize the QR code so as to extract a URL from the recognized QR code. Based on the extracted URL, the reader downloads information from a publisher's server.    [Patent Document 1] Japanese Patent Application Laid-Open No. 11-279474    [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-296659